In the years ahead, the use of domestic coal reserves for energy is expected to become more and more economical and popular. Such use is critical if we are to relieve our dependency on imported petroleum and other liquid/gas fuels. In the past, coal has been shipped by railway, barge and trucks. Such methods of shipping suffice for relatively small quantities of coal. However, as the use of coal increases, it becomes more and more important to provide more economic methods to transport this fuel.
For many years, petroleum and petroleum products have been transported through pipe lines over hundreds and hundreds of miles. The pipe lines are capable of transporting massive volumes of fuel to the urban centers of the country at a fraction of the cost of other transportation modes. It, therefore, is not surprising that coal slurry pipe lines for transporting coal suspended in water or other carrier liquids have been proposed. In fact, many successful slurry pipe lines are in operation today. They, however, are mostly for conveying coal over a relatively short distance to a power plant from an adjacent mine.
A major drawback of a slurry pipeline is the abrasive effect of the solid coal and rock particles on the pump. The particles tend to intrude and lodge between the pumping cylinder and the sealing lip of the piston. The lodged particles can quickly damage the flexible seals and may, under certain conditions, quickly wear the cylinder to the point where the piston no longer seals properly. This results in the loss of pumping pressure. The pump must then be rebuilt, such as by inserting a new cylinder liner and replacing the seals. This repair and replacement of the cylinder liner and seals obviously greatly increases the cost of operation of slurry pumps. Thus, solving this problem would enhance the economic feasibility of slurry pipelines.
The most successful proposal in the past includes the concept of injecting a flushing liquid behind the piston in the pump. As the piston moves forward on the power stroke, the liquid sweeps around the seals on the piston to dislodge the solid particles before damage to the seals or cylinder liner can occur. This past proposal is set forth and claimed in my prior U.S. Pat. No. 4,476,771, issued Oct. 16, 1984.
More specifically, the piston assembly in my prior device includes a first or pumping piston and a second piston coupled together in tandem. Flushing liquid is drawn into a variable volume chamber between the pistons during the suction or return stroke and ejected around the sealing periphery of the pumping piston on the slurry side during the power stroke to prevent particle intrusion of the seal. While this piston assembly structure provides greatly improved self-flushing function, I have recognized the need for even better pumping efficiency while using the same self-flushing piston assembly concept. It has occurred to me that substantial improvement can be gained if the principle of self-flushing slurry pumping could be made to be double-acting; this is with a power stroke in both directions of movement of the piston assembly.